I look at a lot of older games all the way from Amiga to Sega Genesis to Arcade.

I also break down games, record videos, analyze animation frames in photoshop, study sprite sheets, and do experiments and sketches.

I study whatever games I think mine will play like. There's quite a long list because each game has some aspect that I like. For example, maybe I like the simple walk animation in one game, or maybe I like the sword play mechanics in a game. I use them for reference, not to copy, but to have a starting point.